With the widespread deployment DSL (Digital Subscriber Line) transmission for Internet access, there is considerable interest in qualifying candidate telephone lines for high-speed digital transmission. A telephone line is known in the industry as a subscriber loop and it connects a telephone customer to a local switching center known as a central office (CO). The subscriber loop is typically composed of 26 gauge cable but it may contain sections of 24 or 22 gauge cable when there is need to avoid excessive attenuation of the voice signal. Operating practices of telephone service providers have typically resulted in one or more bridge taps near the customer end of the subscriber loop. These bridge taps are open circuit stubs (branches) connected in parallel with the main transmission line and their purpose is to provide flexibility for adding and removing station sets as service demand changes. Bridge taps are a significant impediment to high-speed digital transmission.
Another impediment to high-speed digital transmission is the insertion of loading coils. These 88 mH coils are connected in series with the line at regular intervals and reduce voice frequency attenuation by almost one-half. Loading coils are found on subscriber lines that are longer than about 4 km. These coils, together with the capacitance of the transmission line, form a low pass filter that eliminates the higher frequencies required for high-speed digital transmission.
The maximum transmission range of DSL modems is typically specified at 5.5 km and this would normally allow more than 95% of telephone customers to obtain high-speed Internet access. However, telephone service providers are reluctant to offer high-speed DSL service to customers with loops longer than 4 km since some loop structures may prevent transmission of a satisfactory DSL signal. Consequently, some 30-40% of potential customers (those beyond 4 km) are denied service. Accordingly, there is need to test these long loops to see if they might qualify for future provision of DSL service.
It is desirable to make measurements at the central office or possibly a field cabinet where several hundred -lines are connected to trunk cables. From these measurements it is possible to estimate the structure of a subscriber loop and, based on the estimated loop configuration, it is then possible to estimate the DSL transmission characteristics. It is particularly desirable to conduct these measurements using a single-ended approach at the central office so that the tests can be automated.
One single-ended test method uses a time domain reflectometer (TDR) instrument. A pulse is transmitted and the composition of the loop is estimated using identifiable characteristics in the echo response of the loop. This is a well-established method and there are several commercial test instruments based on this method. A problem with this method is the reduced resolution at long distances caused by spreading of the echo pulses A second single-ended method, and the subject of our invention, measures the return signal over a wide range of transmitted sinusoidal frequencies. This approach is generally referred to as the frequency domain or swept-frequency approach and our instrument will be henceforward referred to as a wideband frequency domain reflectometer (W-FDR). One advantage of the W-FDR method is improved resolution and the ability to discern structural discontinuities that are closely spaced.
Reference is made to the following prior art documents which show arrangements which are relevant to the methods Claimed herein, the disclosures of which are all incorporated herein by reference:
G. J. Erker, D. E. Dodds and W. Krzymien, 1995. “ISDN Loop Extension using a Mid-Span Amplifier”, International Journal of Communication Systems, May-June, Vol. 8, No. 3, pp. 219-224.
U.S. Pat. No. 3,751,606 (Kaiser) issued Aug. 7th 1973, U.S. Pat. No. 3,904,830 (Peoples) issued Sep. 9th 1975 and pending U.S. application 2002/0146095 (Peoples) published Oct. 10th 2002 all assigned to Bell Telephone Labs in which faults are detected by a frequency-domain detection system.
Also in pending U.S. patent application 2004/0062361 (Kamali et al) published Apr. 1st 2004 is disclosed further developments of this same technique.
These patents thus show a method including the following steps:
inputting into one end of the transmission line a sinusoidal signal varied over a range of frequencies;
separating a reflected return signal from the transmitted signal;
and measuring the portion of the reflected signal that is in-phase with the transmitted signal as a function-of frequency applied at said one end to form a “trace”,
and where the Fourier transform is used to generate a spectral analysis of the measured in-phase return signal versus frequency;
and where distances to the irregularities are estimated from the spectral position of peaks in the spectral analysis.